1. Field of the Invention
The invention relates generally to a variable valve timing system, and, more specifically, to a variable valve timing system that uses an electric motor as an actuator.
2. Description of the Related Art
A variable valve timing (VVT) system that changes the phase (i.e., crank angle), at which an intake valve or an exhaust valve is opened/closed, based on the engine operating state has been used. Such variable valve timing system changes the phase of the intake valve or the exhaust valve by rotating a camshaft, which opens/closes the intake valve or the exhaust valve, relative to, for example, a sprocket. The camshaft is rotated hydraulically or by means of an actuator, for example, an electric motor.
With, especially, a variable valve timing system that hydraulically drives a camshaft, the variable valve timing control is sometimes not executed as accurately as it should be, in a cold environment or at the time of engine starting. Such inconvenience is caused because the hydraulic pressure used to drive the camshaft is insufficient or the response of the camshaft to the hydraulic control is slow in such occasions. To obviate such inconveniences, a variable valve timing system that drives a camshaft by means of an electric motor has been suggested, as described in, for example, Japanese Patent Application Publication No. JP-05-104978 (JP-A-05-104978), US Patent Application Publication No. US2005/0252469A1, Japanese Patent Application Publication No. JP-2004-190663 (JP-A-2004-190663).
JP-A-05-104978 describes a variable valve timing system that uses a step motor as an actuator. The described variable valve timing system is provided with a VVT ECU (Electronic Control Unit) that controls the variable valve timing system in addition to an engine ECU that controls an engine. The VVT ECU operates independently of the engine ECU. US Patent Application Publication No. US2005/0252469A1 describes a system (i.e.; a variable valve timing control system) that controls the rotational angle of a camshaft with respect to a crankshaft, using a motor as an actuator. The described system is provided with a control unit for an electric motor that serves as an actuator for the variable valve timing system (i.e., a control unit for the variable valve timing system), in addition to an engine control element. The control unit for the electric motor operates independently of the engine control element.
JP-A-2004-190663 describes a variable valve timing system that changes the valve timing by changing the motor speed with respect to the rotational speed of a camshaft.
However, in the system described in JP-A-05-104978, the VVT ECU calculates an appropriate amount of valve overlap, namely, the appropriate valve timing, upon reception of a signal indicating the operating state of the engine, and also, outputs a control signal for the step motor to achieve the appropriate valve timing. The VVT ECU produces a signal used to select a magnetizing coil of the step motor. Accordingly, the VVT ECU serves many functions including setting of the target value of the valve timing and creation of a control signal which is used to control the step motor to execute the valve timing control appropriately. This increases the processing load placed on the VVT ECU, which is designed to execute software processing by executing a program. Therefore, the processing power of the ECU needs to be increased to control the variable valve timing at a higher speed.
With the system described in US Patent Publication US2005/0252469A1, a signal indicating the rotational angle of the crankshaft and a signal indicating the rotational angle of the camshaft are transmitted to the variable valve timing control unit. The variable valve timing control unit executes a feedback control to match the actual valve timing with the desirable valve timing. This may increase in the processing load placed on the control unit for the variable valve timing system, as in the case of the variable valve timing system described in Japanese Patent Application Publication No. JP-A-05-104978.
According to JP-A-2004-190663, an ECU, which serves as an engine control circuit, calculates the target valve timing and the actual valve timing, and calculates a motor control value (for example, the voltage applied to the motor and value of the final duty ratio for the motor) based on the deviation of the actual valve timing from the target valve timing. In such a configuration, the processing load placed on the ECU that executes the engine control excessively increases. In JP-A-2004-190663, the ECU is designed to execute software processing. Therefore, the processing power of the ECU needs to be increased to control the variable valve timing at a higher speed.
Furthermore, in variable valve timing control described in the above publications, the feedback loop of the valve timing phase control (the control over the position of the valve) is formed for the total system including even the motor control. Accordingly, the control speed may be increased if refinements are introduced to the configuration of the feedback loop.